Let's assume that tab equals 8 spaces
Let's draw characters randomly
Let's allow user to make mistakes

Now we'll define numbers

Zero is
	a large circle

One is
	either
		a long vertical line
	or
		a long vertical line
		a very small slash line
		top of first joins top of second

Two is
	either
		a part of circle 12 to 6
		a long horizontal line
		6 of first joins left of second
	or
		a part of circle 9 to 5
		a slash line
		a long horizontal line
		5 of first joins top of second
		bottom of second joins left of third

Three is
	either
		a part of circle 11 to 6
		a part of circle 12 to 7
		6 of first joins 12 of second
	or
		a long horizontal line
		a slash line
		a part of circle 11 to 7
		right of first joins top of second
		bottom of second joins 11 of third
		

Four is
	either
		a small slash line
		a long horizontal line
		a small vertical line
		bottom of first joins left of second
		second intersects third
	or
		a small vertical line
		a long horizontal line
		a long vertical line
		bottom of first joins left of second
		right of second joins middle of third

Five is
	a long horizontal line
	a small vertical line
	a part of circle 11 to 7
	left of first joins top of second
	bottom of second joins 11 of third
	
Six is
'	either
'		a six-shape
'	or
		a small slash line
		a small circle
		bottom of first joins 10 of second
		
Seven is
	either
		a long horizontal line
		a long slash line
		right of first joins top of second
	or
		a long horizontal line
		a long slash line
		a very short horizontal line
		right of first joins top of second
		third intersects second

Eight is
'	either
'		an eight-like-shape
'	or
		a small circle
		a small circle
		6 of first joins 12 of second
		
Nine is
'	either
'		a nine-shape
'	or
		a small circle
		a small slash line
		5 of first joins top of second

So now all numbers are covered and we'll describe operators

Plus is
	a horizontal line
	a vertical line
	first intersects second

Multiply is
	a short slash line
	a short backslash line
	first intersects second

Divide is
	a slash line

Minus is
	a horizontal line

Equals is
	a horizontal line
	a horizontal line
	first never intersects second

Finally there are letters that can't be entered by user but may be present in output

LetterE is
	a long vertical line
	a long horizontal line
	a long horizontal line
	a long horizontal line
	left of second joins top of first
	left of third joins middle of first
	left of fourth joins bottom of first
	
LetterR is
	either
		a short vertical line
		a long horizontal line
		top of first joins left of second
	or
		a short vertical line
		a part of small circle 9 to 12
		middle of first joins 9 of second 
		
Oh, I've almost forgotten to include decimal point

Dot is
	a very small circle

That's all!